Process for isomerizing C5-C8 paraffin cuts rich in paraffins containing more than seven carbon atoms

ABSTRACT

A process for isomerising a feed comprising normal paraffins containing 5 to 8 carbon atoms per molecule as a major portion in the presence of hydrogen is characterized in that the sum of the amounts of normal paraffins containing 7 and 8 carbon atoms per molecule contained in the feed is in the range 2% to 90% by weight with respect to the feed, and in that said feed is treated in at least one reaction zone containing at least one catalyst in a fixed bed, said catalyst comprising a support, at least one halogen and at least one group VIII metal, the reaction being carried out at a temperature in the range 30° C. to 150° C.

[0001] The present invention relates to a process for isomerisation, inthe presence of hydrogen (sometimes also known as a hydroisomerisationprocess) of a feed comprising normal paraffins (also termed n-paraffinsor normal paraffins) containing 5 to 8 carbon atoms per molecule as amajor portion.

[0002] The removal of lead alkyls from automobile gasoline forenvironmental protection reasons has prompted the development ofprocesses for producing branched chain paraffins which have a betteroctane number than linear compounds, in particular a process forisomerising normal paraffins to branched paraffins. The importance ofthis process to the petroleum industry id currently increasing.

[0003] Isomerising n-butane (normal butane) leads to the production ofisobutane which can be used in different applications. Examples of suchapplications are: processes for alkylating light olefins to produceparaffinic cuts containing 5 to 12 carbon atoms per molecule. Alkylationis carried out using at least one isoparaffin, and it can produce cutswith high octane numbers. After dehydrogenation, isobutane can also beused to etherify methanol or ethanol. The ethers obtained—methyltertio-butyl ether (MTBE) or ethyl tertio-butyl ether (ETBE) have highoctane numbers and can be directly incorporated into the gasoline.

[0004] The process for isomerising paraffins containing 5 and 6 carbonatoms per molecule can also lead to the production of high octane numbergasoline bases which can be directly incorporated into the gasolinefractions. The latter process has been the subject of numerous studies,and three different catalyst types have traditionally been used to carryout the isomerisation reaction:

[0005] Friedel-Crafts type catalysts, such as catalysts containingaluminium chloride, which are used at low temperatures (about 20° C. to130° C.);

[0006] catalysts based on metals from group VIII of the periodic table(“Handbook of Chemistry and Physics”, 45^(th) Edition, 1964-1965)deposited on alumina and generally containing a halogen, which are usedat medium temperatures (about 110° C. to 160° C.). United States patentsU.S. Pat. No. 2,906,798, U.S. Pat. No. 2,993,398, U.S. Pat. No.3,791,960, U.S. Pat. No. 4,113,789, U.S. Pat. No. 4,149,993, U.S. Pat.No. 4,804,803, for example, describe such catalysts;

[0007] zeolitic catalysts comprising a group VIII metal deposited onzeolite, used at high temperatures (250° C. to 350° C.); such catalystslead to the production of a mixture of hydrocarbons with an octanenumber which is improved but is not as good as that obtained byprocesses using the catalysts cited above. However, they have theadvantage of being easier to use and more resistant to poisons. The lowacidity does not enable them to be used to isomerise n-butane. U.S. Pat.No. 4,727,217 describes such catalysts.

[0008] Current processes for isomerising paraffins containing 5 and 6carbon atoms using chlorinated alumina type catalysts and includingplatinum are high activity catalysts. Such processes are used oncethrough or with partial recycling after fractionating the unconvertednormal paraffins, or with complete recycling after passage throughmolecular sieve systems in the liquid phase. Such processes lead to theproduction of a base for fuels containing no aromatic compounds and witha research octane number (RON) generally in the range 82 to 88, thenormal paraffin isomerisation process including or not includingrecycling.

[0009] Numerous patents have concerned monometallic platinum basedcatalysts deposited on a halogenated alumina, and their use in normalparaffin isomerisation processes. U.S. Pat. No. 3,963,643 describestreatment with a Friedel-Crafts type compound followed by treatment witha chlorinated compound containing at least two chlorine atoms, suchtreatment more particularly being applicable to straight chainhydrocarbons containing 4 to 6 carbon atoms. U.S. Pat. No. 5,166,121describes a catalyst comprising gamma alumina in the form of beads andcomprising 0.1% to 3.5% of halogen on the support. The amount of halogen(preferably chlorine) deposited on the support is extremely small.

[0010] As is currently publicly -known, a major drawback of suchprocesses is that they feeds containing more than about 2% by weight ofnormal paraffins containing at least 7 carbon atoms per molecule cannotbe treated properly. Operating conditions which are known to encourageisomerisation of cuts comprising paraffins containing 5 and 6 carbonatoms per molecule leads to degrees of cracking for paraffins containingat least 7 carbon atoms per molecule and which are too large (of theorder of 20% to 80% for C7 paraffins).

[0011] An examination of the prior art shows that catalysts forisomerising normal paraffins containing 7 carbon atoms have beenstudied. In such processes, the reaction temperature is generally over200° C. and normally over 300° C. and the ratio of the number of molesof hydrogen over the number of moles of hydrocarbons is more than 1.Such operating conditions do not encourage the production of highlybranched paraffins. Thermodynamic equilibrium data indicate that theamount of paraffin branching reduces as the temperature increases.

[0012] French patent FR-A-2 735 993 describes a catalyst and its use inprocesses for isomerising normal paraffins containing 4 to 6 carbonatoms. The catalyst contains at least one halogen, preferably chlorine,at least one group VIII metal and a formed support comprising gammaalumina and/or eta alumina, the catalyst being characterized in that thesmallest average dimension of said support is about 0.8 to 2 mm,preferably about 1 to 1.8 mm, and in that its chlorine content is about4.5% to 15% by weight, preferably about 5% to 12% by weight. Thecatalyst is prepared by halogenating a catalyst containing at least onegroup VIII metal on an alumina support. Once the metal has beendeposited, the support can be activated in air and/or in nitrogen.

[0013] A halogenated catalyst can also be prepared from a support whichis formed and steam treated. Such a catalyst forms the subject matter ofa patent application by the Applicant, filed on the same day as thepresent application, which describes a catalyst containing at least onehalogen, at least one group VIII metal and a formed support comprisinggamma alumina and/or eta alumina, treated in a stream of gas containingsteam.

[0014] The present invention provides a process for isomerising a feedcomprising normal paraffins containing 5 to 8 carbon atoms per moleculeas a major portion in the presence of hydrogen, characterized in thatthe sum of the amounts of normal paraffins containing 7 and 8 carbonatoms per molecule contained in the feed is in the range 2% to 90% byweight with respect to the feed, and in that said feed is treated in atleast one reaction zone containing at least one catalyst in a fixed bed,said catalyst comprising a support, at least one halogen and at leastone group VIII metal, the reaction being carried out at a temperature inthe range 3⁰° C. to 150° C..

[0015] The present invention also provides a process for increasing theoctane number of a petroleum cut comprising normal paraffins containing5 to 8 carbon atoms per molecule. In particular, the present inventionovercomes the above disadvantages. The process of the invention canconvert feeds in which the sum of normal paraffins containing 7 and 8carbon atoms per molecule contained in said feed is in the range 2% to90% by weight, preferably in the range 5% to 90% by weight, morepreferably in the range 20% to 90% by weight, and very preferably in therange 40% to 90% by weight. The process of the present invention canproduce a yield of branched paraffins containing at least 5 carbon atomsper molecule of over 85% by weight from a feed to be treated comprisingnormal paraffins containing 5 to 8 carbon atoms per molecule,.

[0016] The process of the present invention uses at least one reactionzone which comprises at least one reactor preferably containing at leastone solid acid catalyst in a fixed bed, the reaction temperature beingin the range 30° C. to 150° C., preferably in the range 70° C. to 130°C., more preferably in the range 70° C. to 95° C. The catalyst usedcomprises a support, preferably an alumina based support, containing atleast one halogen, the halogen content being in the range 0.1% to 15% byweight, and at least one group VIII metal. In a preferred embodiment ofthe invention, a chlorinated alumina based catalyst is used.

[0017] The catalyst used in the process of the invention contains atleast one group VIII metal on a support, preferably an alumina basedsupport, and on the support at least one halogen is deposited,preferably selected from the group formed by fluorine, chlorine, bromineand iodine; more preferably the halogen is chlorine. The halogen contentis in the range 0.1% to 15% by weight, preferably in the range 4% to 12%by weight. The catalyst support preferably essentially comprisesalumina. The alumina which is preferably used in the process of theinvention can be gamma alumina and/or, possibly, eta alumina (i.e.,constituted either by gamma alumina, or eta alumina, or a mixture ofthese two aluminas). When gamma alumina is added to eta alumina, thealumina of the support comprises between 50% and 100% by weight,preferably between 80% and 100% by weight, of eta alumina, morepreferably 80% to 95% by weight of eta alumina, the complement beinggamma alumina.

[0018] The smallest average dimension of the catalyst support is about0.8 to 2 mm, preferably about 1 to 1.8 mm. The support is preferablyessentially formed from beads with an average diameter of about 0.8 to 2mm, preferably about 1 to 1.8 mm, or the support is essentially formedfrom extrudates with a smallest dimension of about 0.8 to 2 mm,preferably about 1 to 1.8 mm, i.e., the extrudates are formed using anyextrusion technique known to the skilled person, such as a die with adiameter of about 0.8 to 2 mm, preferably about 1 to 1.8 mm.

[0019] The gamma alumina possibly present in the catalyst support has aspecific surface area of about 150 to 300 m²/g and preferably about 180to 250 m²/g, and a total pore volume of about 0.4 to 0.8 cm³/g,preferably about 0.45 to 0.7 cm³/g.

[0020] The eta alumina which is optionally present in the catalystsupport has a specific surface area of about 400 to 600 m²/g, preferablyabout 420 to 550 m²/g, and a total pore volume of about 0.3 to 0.5cm³/g, preferably about 0.35 to 0.45 cm³/g.

[0021] The group VIII metal is selected from the group formed by iron,cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium andplatinum, preferably selected from the group formed by platinum,palladium and nickel. In the preferred case where said metal is platinumor palladium, the content is about 0.05% to 2% by weight, preferablyabout 0.1% to 1.5% by weight. In the preferred case where said metal isnickel, the content is about 0.1% to 10% by weight, preferably about0.2% to 6% by weight.

[0022] The catalyst is generally prepared by forming the support. Theformed support can optionally be steam treated at high temperaturebefore or after depositing at least one group VIII metal. Halogenation,preferably chlorination, is then carried out. It is also possible, andpreferred, to carry out an activation step in hydrogen before saidhalogenation step. Each step of the process for preparing the support ofthe invention will be explained below.

[0023] When two types of alumina (gamma and eta) are present in thecatalyst support, these two types of aluminas are preferably mixed andformed together using any technique which is known to the skilledperson, for example by extrusion through a die, pelletization or bowlgranulation. However, it is also possible to form the two types ofalumina separately then to proceed to mixing the two types of formedalumina. In all cases, the smallest dimension of the geometric shapedescribed by the support after forming is about 0.8 to 2 mm, preferablyabout I to 1.8 mm, in order to produce a sufficient halogen content fora reduced halogenation period during the support halogenation step.

[0024] The support preferably undergoes high temperature treatment usingsteam. The hydrothermal treatment is generally carried out for 0.5 to 6hours, for example, at a temperature of about 200° C. to 700° C. in astream of gas, for example air and/or nitrogen. The gas must containwater, for example in an amount of about 0.2% to 100% by volume andpreferably about 0.3% to 20% by volume. Activation of the alumina by thesteam can produce much more acidic catalysts which are thus more activefor isomerisation.

[0025] At least one hydrogenating metal from group VIII selected fromthe group formed by iron, cobalt, nickel, ruthenium, rhodium, palladium,osrmium, iridium and platinum, preferably selected from the group formedby platinum, palladium, and nickel, is then deposited on the supportusing any technique which is known to the skilled person, for example byanion exchange using hexachloroplatinic acid when using platinum or inthe form of the chloride when using palladium. The hydrothermaltreatment can also be carried out after depositing the metal on thesupport.

[0026] The support comprising the deposited metal can thus optionallyundergo a treatment in hydrogen to produce an active metallic phase. Theprocedure of this treatment under hydrogen comprises, for example, aslow rise in temperature in a stream of hydrogen up to the maximumreduction temperature which is about 300° C. to 700° C., preferably inthe range 340° C. to 680° C., followed by holding that temperaturegenerally for 1 to 6 hours, preferably for 1.5 to 4.5 hours.

[0027] The halogenation step can be carried out using any techniqueknown to the skilled person. The halogen, preferably chlorine, ispreferably deposited from any carbon-containing compound also containinghalogen atoms and known to allow halogenation, preferably underconditions which the skilled person would judge to be suitable havingregard to treatment of effluents, the halogenation duration or the cost.For this reason, the hydrogen chloride is rarely if ever used. Aluminais halogenated, preferably chlorinated, directly in an isomerisationunit before injecting the feed to be treated, or offsite: in a separateunit provided for halogenation. Halogenation can be carried out usingany halogenating carbon-containing agent, preferably a chlorinatingagent, which is known to the skilled person. In the preferred case whenthe halogen is chlorine, carbon tetrachloride or chloroform are normallyused.

[0028] In the catalyst preparation process, it is also possible to carryout the halogenation treatment prior to reduction in hydrogen. In thiscase, reduction in hydrogen can take place outside the unit (ex situ),meaning that particular precautions must be taken when transporting thecatalyst to that unit, or treatment can take place in the unit (insitu), just before the catalyst is used.

[0029] The present invention provides a process for isomerising a feedcomprising normal paraffins containing 5 to 8 carbon atoms per moleculeas a major portion, characterized in that the sum of the amounts ofnormal paraffins containing 7 and 8 carbon atoms per molecule containedin the feed is in the range 2% to 90% by weight, preferably in the range5% to 90% benzene, more preferably in the range 20% to 90% by weight,and highly preferably in the range 40% to 90% by weight with respect tothe feed, and in that said feed is treated in at least one reactionzone, preferably containing at least one catalyst in a fixed bed, saidcatalyst comprising a support, at least one halogen and at least onegroup VIII metal, the reaction being carried out at a temperature in therange 30° C. to 150° C., preferably 70° C. to 130° C., more preferablyin the range 70° C. to 95° C., the feed to be treated preferablycontaining at least one halogenated compound, more preferably achlorinated compound, the amount by weight in said feed being in therange 50 to 2000 ppm, usually 50 to 300 ppm, for exampleperchloroethylene C₂Cl₄.

[0030] Two implementations of the invention can be considered, selecteddepending on the amount of excess hydrogen with respect to the quantityof hydrogen consumed by hydrogenation, naphthene ring opening andparaffin cracking reactions. This can also be expressed as the ratio Rof the number of moles of hydrogen over the number of moles ofhydrocarbons in the effluent leaving the reactor.

[0031] In the first implementation of the invention, a slight excess ofhydrogen is used, such that the ratio R of the number of moles ofhydrogen over the number of moles of hydrocarbon calculated on the basisof the composition of the effluent leaving the reactor is in the range0.06 to 0.3, preferably in the range 0.06 to 0.2. In this case it is notnecessary to recycle non consumed hydrogen to the reactor inlet. This isthus a “lost hydrogen” operation.

[0032] In the second implementation of the process of the invention, alarge excess of hydrogen is used. The ratio R of the number of moles ofhydrogen over the number of moles of hydrocarbon calculated on the basisof the composition of the effluent leaving the reactor is thus in therange 0.3 to 10, preferably in the range 0.3 to 5, and more preferablyin the range 0.5 to 3. In this case the excess hydrogen is recycled tothe reactor inlet, for example by means of a gas-liquid separation drumand a recycling compressor. In this implementation of the invention, therange of adjustment for the partial pressure of the hydrogen is widerthan in the first implementation.

[0033] The preferred ranges given below for the operating conditions areapplicable to both implementations of the process of the invention.

[0034] The hourly space velocity (HSV), defined as the mass flow rate offeed to be treated per mass of catalyst per hour, is about 0.2 to 10 kgof feed per kg of catalyst per hour (0.2 to 10 h⁻¹), preferably about0.3 to 5 kg of feed per kg of catalyst per hour (0.3 to 5 h⁻¹), and morepreferably about 0.5 to 2 kg of feed per kg of catalyst per hour (0.5 to2 h⁻¹).

[0035] The reactor pressure is about 0.1 to 10 MPa relative, preferablyabout 0.5 to 8 MPa relative, more preferably between 2 and 5 MPa.

[0036] The reactor temperature is in the range 30° C. to 150° C.,preferably in the range 70° C. to 130° C., more preferably in the range70° C. to 95° C.

[0037] The use of a catalyst comprising a support, at least one halogenand at least one group VIII metal under the operating conditionsdescribed above surprisingly leads to high C5-C8 n-paraffin conversionlevels, more particularly n-heptane, while keeping yields of theisomerates high, i.e., the yields of light gasoline essentiallyconstituted by hydrocarbons containing 5 to 8 carbon atoms. The use ofthis catalyst thus keeps the degree of cracking low.

[0038] The process of the invention can be used to treat all types offeeds comprising, as a major portion, normal paraffins containing 5 to 8carbon atoms, naphthenes, and aromatic compounds (normally in quantitiesof less than 10% by weight). More particularly, the process of theinvention can be used to treat paraffin cuts with a chain containing 5to 8 carbon atoms, and in which the sum of the amounts of normalparaffins containing 7 and 8 carbon atoms per molecule comprised in thecut is in the range 2% to 90% by weight, preferably in the range 5% to90% by weight, more preferably in the range 20% to 90% by weight, andhighly preferably in the range 40% to 90% by weight.

[0039] Preferably, care is taken that feeds for the process of theinvention are free of water, oxygen, sulphur and more generally of anyknown compounds which are known to be poisons or inhibitors forcatalysts based on halogenated alumina.

[0040] The following examples illustrate the invention without in anyway limiting its scope.

EXAMPLES

[0041] The volume of the reactor used was 200 ml, operated in riser modewith the fluids supplied being a mixture constituted by the feed to betreated and hydrogen. The effluent leaving the reactor was cooled thenanalysed by gas chromatography.

Example 1 (in accordance with the invention)

[0042] In this Example, an industrial catalyst based on chlorinatealumina sold by Procatalyse, reference number IS 612A, was used.

[0043] The volume of the reactor used was 200 ml, operated in riser modewith the fluids supplied being a mixture constituted by the feed to betreated and hydrogen. The effluent leaving the reactor was cooled thenanalysed by gas chromatography.

[0044] The operating conditions were as follows:

[0045] The reactor was fed with a feed comprising hydrocarbonscontaining 5 to 7 carbon atoms and 800 ppm by weight ofperchloroethylene (C₂Cl₄) at a flow rate of 87 g/h, the catalyst massbeing 86 g, the HSV 1.01 h⁻¹. The hydrogen flow rate was 4.5×10⁻⁹ 1/h.The total pressure was 3 MPa relative. Two isomerisation steps werecarried out using the same feed at different temperatures. Isomerisation1 was carried out at a temperature of 105° C.: the ratio R1 of thenumber of moles of hydrogen over the number of moles of hydrocarbons,calculated at the reactor outlet, was 0.14; isomerisation 2 was carriedout at a temperature of 115° C.: the ratio R2 of the number of moles ofhydrogen over the number of moles of hydrocarbons, calculated at thereactor outlet, was 0.11.

[0046] The results obtained are shown in Table 1. TABLE 1 After AfterFeed isomerisation 1 isomensation 2 Compounds (wt %) (wt %) (wt %) C2-C40.74 5.19 7.15 iC5 4.19 6.72 7.62 nC5 10.53 7.67 7.5 Cyclopentane 0.280.27 0.28 iC6 4.01 4.32 4.73 nC6 1.06 0.82 0.88 Cyclohexane 1.4 3.04 2.6Methylcyclopentane 1.01 1.66 1.62 Benzene 0.01 0 0 nC7 65.7 20.00 17.35iC7 11.7 50.31 50.27 Isomerisation 1 Isomerisation 2 nC5 conversion  29%   27% nC6 conversion   17% 22.60% nC7 conversion 73.6% 69.60% C5+yield 93.5% 95.50%

[0047] The results of Table 1 show that n-heptane conversion levels ofthe order of 70% were obtained, while producing only 4.45% by weight oflight products for isomerisation carried out at 105° C. and 6.41% byweight for isomerisation carried out at 115° C. The term “lightproducts” means a fraction essentially constituted by hydrocarbonscontaining 2 to 4 carbon atoms.

[0048] These results are of great industrial importance as the operatingconditions were very mild: temperatures of 105° C. and 115° C.

Example 2 (in accordance with the invention)

[0049] This example used the same catalyst and reactor as that ofExample 1.

[0050] The reactor was supplied with a feed comprising hydrocarbonscontaining 5 to 7 carbon atoms and 800 ppm of perchloroethylene (C₂Cl₄)at a rate of 84 g/h, the catalyst mass being 84 g, the HSV being 1 h⁻¹.The hydrogen flow rate was 60×10⁻⁹ 1/h. The total pressure was 3 MParelative.

[0051] Two isomerisation steps were carried out using the same feed atdifferent temperatures. Isomerisation 3 was carried out at a temperatureof 115° C., the ratio R3 of the number of moles of hydrogen over thenumber of moles of hydrocarbons, calculated at the reactor outlet, was2.67; isomerisation 4 was carried out at a temperature of 130° C.: theratio R4 of the number of moles of hydrogen over the number of moles ofhydrocarbons, calculated at the reactor outlet, was 2.56.

[0052] The principal difference with respect to Example 1 was thatExample 2 corresponded to an isomerisation process in which a largeexcess of hydrogen with respect to the feed to be converted was used.

[0053] The composition of the feed and the results obtained are shown inTable 2. TABLE 2 After After Feed isomerisation 3 isomerisation 4Compounds (wt %) (wt %) (wt %) C2-C4 0.87 5.99 9.51 iC5 9.95 11.73 12.5nC5 7.79 6.33 6.18 Cyclopentane 0.62 0.62 0.62 iC6 9.50 10.40 11.01 nC62.97 2.07 2.02 Cyclohexane 5.10 3.79 3.19 Methylcyclopentane 2.32 2.472.67 Benzene 0.17 0 0 nC7 55.41 13.63 9.15 iC7 5.30 42.97 43.15Isomerisation 1 Isomerisation 2 nC5 conversion 18.8% 20.7% nC6conversion 30.3%   32% nC7 conversion 75.4% 83.5% CS+ yield 94.8% 91.3%

[0054] As in Example 1, high degrees of conversion of n-heptanes toiso-heptanes were obtained under operating conditions in which thequantity of light products formed by cracking remained low. The resultsshown in Table 2 indicate that degrees of n-heptane. conversion obtainedwere of the order of 75-80%, while only 5.1% by weight of light productswere produced for isomerisation carried out at 115° C. and 8.7% byweight for isomerisation carried out at 130° C.

[0055] Table 2 also shows that 130° C. was a temperature only slightlydifferent from the maximum temperature compatible with the production ofhigh isomerate yields, in particular if it is assumed that the degree ofcracking to light products of 10% was the upper acceptable limit. At130° C., 8.7% of fight products were formed by cracking, giving a yieldof branched paraffins containing 5 to 7 carbon atoms of 91.3%.

Example 3 in accordance with the invention)

[0056] The catalyst used in Example 3 was produced as follows: gammaalumina was formed by extrusion through a 1.2 mm diameter die. The solidobtained was treated at 500° C. with air containing 3% by weight ofsteam. 0.2% of platinum was deposited on this alumina by ion exchangewith hexachloroplatinic acid in the presence of HCI as a competingagent. The solid obtained was reduced in hydrogen at 400° C.

[0057] The solid obtained was then chlorinated at a temperature of 280°C. by injecting carbon tetrachloride in a stream of nitrogen.

[0058] The feed to be treated was constituted by about 10% by weight ofnormal paraffins containing 5 carbon atoms, 10% by weight of normalparaffins containing 6 carbon atoms, 65% by weight of normal paraffinscontaining 7 carbon atoms and 8% by weight of naphthenes containing 6carbon atoms. The feed containing 100 ppm of carbon tetrachloride(CCl₄), expressed as the weight of chlorine, to maintain the amount ofchlorine in the catalyst used.

[0059] The isomensation operating conditions were as follows: thereactor temperature was 110° C., the total pressure was 3 MPa relative,the HSV was 1 h⁻¹ and the ratio R5 of the number of moles of hydrogenover the number of moles of hydrocarbons calculated at the reactoroutlet was 0.47.

[0060] The performances obtained after 24 hours of operation were asfollows: a degree of n-heptane conversion of 73.5, and only 4.6% byweight of light products were produced.

1. A process for isomerising a feed comprising normal paraffinscontaining 5 to 8 carbon atoms per molecule as a major portion in thepresence of hydrogen, characterized in that the sum of the amounts ofnormal paraffins containing 7 and 8 carbon atoms per molecule containedin the feed is in the range 2% to 90% by weight with respect to thefeed, and in that said feed is treated in at least one reaction zone,containing at least one catalyst in a fixed bed, said catalystcomprising a support, at least one halogen and at least one group VIIImetal, the reaction being carried out at a temperature in the range 30to 150° C.
 2. A process according to claim 1, in which the support isalumina based.
 3. A process according to claim 1 or claim 2, in whichthe feed to be treated contains at least one halogenated compound in anamount in said feed in the range 50 to 2000 ppm by weight.
 4. Anisomerisation process according to any one of claims 1 to 3,characterized in that the halogen contained in the support is chlorine.5. An isomerisation process according to any one of claims 1 to 4,characterized in that a treatment of the support at high temperature insteam is carried out before or after depositing at least one metal. 6.An isomerisation process according to claim 5, in which the support istreated for 0.5 to 6 hours at a temperature of about 200° C. to 700° C.,in a stream of a gas containing water in amounts of about 0.2% to 100%by volume.
 7. An isomerisation process according to any one of claims 1to 6, in which the sum of the amounts of normal paraffins containing 7and 8 carbon atoms per molecule contained in the feed is in the range 5%to 90% by weight.
 8. An isomerisation process according to any one ofclaims 1 to 6, in which the sum of the amounts of normal paraffinscontaining 7 and 8 carbon atoms per molecule contained in the feed is inthe range 20% to 90% by weight.
 9. An isomerisation process according toany one of claims 1 to 8, characterized in that the support contains ahalogen in amounts in the range 0.1% to 15% by weight.
 10. Anisomerisation process according to any one of claims 1 to 9,characterized in that the total reaction pressure is about 0.1 to 10 MParelative, the hourly space velocity being about 0.2 to 10 h⁻¹.
 11. Anisomerisation process according to any one of claims 1 to 10,characterized in that the reaction is carried out in the presence of anexcess of hydrogen such that the ratio R of the number of moles ofhydrogen over the number of moles of hydrocarbons calculated on thebasis of the composition of the effluent leaving the reactor is in therange 0.06 to 0.3.
 12. An isomerisation process according to any one ofclaims 1 to 10, characterized in that the reaction is carried out in thepresence of an excess of hydrogen such that the ratio of the number ofmoles of hydrogen over the number of moles of hydrocarbons calculated onthe basis of the composition of the effluent leaving the reactor is inthe range 0.3 to
 10. 13. An isomerisation process according to any oneof claims 1 to 12, characterized in that the catalyst undergoestreatment in hydrogen before depositing at least one halogen.
 14. Anisomerisation process according to claim 13, characterized in that thetreatment in hydrogen comprises a slow rise in temperature in a streamof hydrogen up to the maximum reduction temperature which is about 300°C. to 700° C., followed by maintaining that temperature, generally for 1to 6 hours.